Among the so-called third generation communication schemes, commercial service of a W-CDMA (Wideband Code division Multiple Access) scheme has started from 2001 in Japan. In addition, HSDPA (High Speed Downlink Packet Access) service is scheduled to start which implements higher speed data transmission using a downlink by adding a channel (HS-DSCH: High Speed-Downlink Shared Channel) for packet transmission to the downlink (dedicated data channel and dedicated control channel). Furthermore, to further speed up uplink data transmission, an HSUPA (High Speed Uplink Packet Access) scheme has been proposed and investigated. The W-CDMA is a communication scheme determined by 3GPP (3rd Generation Partnership Project), a standardization organization of mobile communication systems, and specifications of a sixth release version have been arranged at present.
In addition, as communication schemes other than W-CDMA, 3GPP investigates new communication schemes referred to as “Long Term Evolution” (LTE) for radio sections and “System Architecture Evolution” (SAE) for the total system configuration including a core network. In LTE, an access scheme, radio channel configuration and protocol differ from those of the current W-CDMA (HSDPA/HSUPA). For example, as for the access scheme, although W-CDMA employs CDMA (Code Division Multiple Access), LTE uses OFDM (Orthogonal Frequency Division Multiplexing) in the downlink direction, and SC-FDMA (Single Carrier Frequency Division Multiple Access) in the uplink direction. In addition, as for a bandwidth, although W-CDMA is 5 MHz, LTE can use 1.25/2.5/5/10/15/20 MHz. Besides, LTE does not employ circuit switching as W-CDMA does, but uses only a packet communication scheme.
Since LTE employs a new core network different from the core network (GPRS) of W-CDMA to construct a communication system, it is defined as a radio access network independent of a W-CDMA network. Accordingly, to distinguish from the W-CDMA communication system, in the LTE communication system, a base station for performing communication with mobile terminal UE (User Equipment) is called eNB (E-UTRAN NodeB), and a base station controller (Radio Network Controller) for transferring control data and user data between it and a plurality of base stations is called an aGW (Access Gateway). The multimedia multicast/broadcast service carried out by the LTE communication system is referred to as E-MBMS (Evolved Multimedia Broadcast Multicast Service), and transmits masses of broadcasting mass contents such as news, weather forecasts and mobile broadcasting to a plurality of mobile terminals. It is also referred to as point to multipoint service. A base station transmits E-MBMS data to mobile terminals by mapping the E-MBMS data on a DL-SCH (Downlink Shared Channel) or on an MCH (Multicast Channel). In addition, LTE provides not only broadcast communication service, but also communication service to each of the plurality of mobile terminals. The communication service for the individual mobile terminals is referred to as Unicast service. Since LTE differs from W-CDMA in that it does not have dedicated channels (Dedicated Channel and Dedicated Physical Channel) for the individual mobile terminals in the transport channel and physical channel, it carries out the data transmission to the individual mobile terminals through a shared channel.
An LTE communication system has two types of transmission modes in the E-MBMS service it provides: a multi-cell transmission mode and a single-cell transmission mode. In the multi-cell transmission, a plurality of base stations transmit the same E-MBMS broadcast service at the same frequency so that a mobile terminal can combine E-MBMS data sent from a plurality of base stations. The E-MBMS data is mapped onto an MCH to be transmitted. On the other hand, in the single-cell transmission, the same E-MBMS broadcast service is transmitted within only one cell. In this case, the E-MBMS data is mapped onto a DL-SCH to be transmitted. In the single-cell transmission, each base station can transmit the E-MBMS data at a different frequency. To enable the mobile terminal to receive the multi-cell transmitted E-MBMS data and to combine the E-MBMS data, it is necessary to suppress inter-symbol interference which is interference between the E-MBMS data transmitted from a plurality of base stations. To handle the foregoing problem, Non-Patent Document 1 discloses a base station that transmits, at the multi-cell transmission, the E-MBMS data in such a manner that the timing difference in the reception by a mobile terminal falls within an OFDM guard interval (referred to as “OFDM guard interval CP (Cycle Prefix)”).
Non-Patent Document 1: 3GPP TR25.912 V7.0.0.
Although the LTE communication system employs OFDM as the access scheme for the downlink transmission, OFDM is considered to be an access scheme that is comparatively weak at interference. Thus, even if a plurality of base stations carry out multi-cell transmission that transmits the same E-MBMS data, the number of the transmitting base stations is preferably determined at an appropriate number from the point of view of the interference suppression. In addition, transmission from an unnecessary base station not only brings about interference, but also is undesirable from the point of view of the effective use of radio resources. Non-Patent Document 2 discloses a communication method of transmitting the E-MBMS data to a mobile terminal that sends a request only from a base station that receives from the mobile terminal the reception request (counting or entry (such as entry, subscribe and activation)) for the E-MBMS service (content) and from the neighboring base stations of the base station. However, Non-Patent Document 2 does not describe a manner of selecting the base stations that transmit the E-MBMS data to the mobile terminal that transmits the reception request.
Non-Patent Document 2: 3GPP R3-061205.
As a technique for a mobile terminal to receive and combine the same data transmitted from a plurality of base stations, there is RAKE combining at a soft handover. The RAKE combining is applied to a communication system using the third-generation W-CDMA access scheme. When the mobile terminal is located close to a region at which the base station is switched, a plurality of base stations in its adjacent cells transmit the same dedicated data (DPDCH: Dedicated Physical Data Channel) to the mobile terminal. In the W-CDMA system, since each base station multiplies a different scramble code, the mobile terminal carries out receiving processing (despreading) separately for the received signal from each of a plurality of base stations to combine the data from the plurality of base stations. For example, the mobile terminal which can receive the data from three base stations and perform RAKE combining of them must have three branches of receiving sections (such as despreading sections) to perform receiving processing of the received data from each base station separately.
At the soft handover, an active set for the soft handover is created as a set of the base stations that transmit the same dedicated data to the mobile terminal. As for the number of articles contained in the active set, although it is variable according to the receiving capability of the mobile terminal (such as the number of base stations that can undergo receiving processing simultaneously) or according to instructions from the communication system, its upper limit is set at six in the present state of things. FIG. 13 is a diagram for explaining the processing of creating the active set for the soft handover. In the graph shown in FIG. 13, the vertical axis represents measurement quality obtained by measuring by the mobile terminal the signal received from the base station, and the horizontal axis indicates time. The mobile terminal measures the powers of the received signals from first to third base stations, thereby measuring reception qualities. At time T4, the curve of the first base station crosses the curve of the second base station, which means that the measurement quality of the second base station exceeds the measurement quality of the first base station at time T4. Broken lines on FIG. 13, which represent the curve of the first base station before time T4 and the curve of the second base station after time T4, indicate the highest measurement quality among the reception qualities of the signals the mobile terminal is receiving from the plurality of base stations. In addition, dash-dotted lines, which denote a curve obtained from the curve indicating the highest measurement quality represented by the broken lines and a “reporting range” of which the network side notifies the mobile terminal, represent the receiving level obtained by subtracting the reporting range from the values of the highest measurement quality (receiving level denoted by the broken lines). The dash-dotted lines on the graph are used as a dynamic threshold for creating the active set for the soft handover.
At time T1 in FIG. 13, the received signals from the first base station and second base station show measurement qualities higher than the threshold denoted by the dash-dotted lines. Accordingly, at time T1, both the first base station and second base station become the base station candidates for the active set for the soft handover. On the other hand, at time T2, since the received signal from the third base station exceeds the threshold denoted by the dash-dotted lines, third base station is added as a new base station candidate for the active set for the soft handover. Here, the mobile terminal transmits an additional event for making the third base station an additional candidate for the active set. At time T3, since the received signal from the third base station falls below the threshold denoted by the dash-dotted lines, the third base station becomes a candidate to be deleted from the base station candidates for the active set for the soft handover. Here, the mobile terminal transmits a delete event for deleting the third base station from the active set. As described above, the threshold for making a decision of the additional or delete base station candidate is obtained by subtracting the reporting range from the highest measurement quality values. The highest measurement quality values vary in accordance with the distance from the base station to the mobile terminal and the like. In other words, the threshold is a dynamic threshold that varies in accordance with the receiving conditions of the mobile terminal. As for the selection of the active set for the soft handover, Non-Patent Document 3 describes it.
Non-Patent Document 3: 3GPP TS25.331 V6.10.0.
Furthermore, a method and a system for performing a handoff by using the dynamic threshold that adaptively varies as described above are described in Patent Document 1. Patent Document 1 discloses that the adaptive dynamic threshold is determined by a function of quality levels of the highest transmission source and the lowest transmission source of the base stations (transmission sources) contained in the active set.
Patent Document 1: Japanese Patent Laid-Open No. 2003-525533.